In related-art, Transmission Electron Microscope (TEM) observation is known as a method of observing a micro region in a sample for analyzing defects in a semiconductor device or other purposes. In TEM observation, when preparing a sample for obtaining a transmission electron image, it is necessary to prepare a TEM sample having a thin film portion, which has such a thickness that allows an electron beam to transmit therethrough, at a portion thereof.
In recent years, as a method of preparing a TEM sample, a TEM sample preparation method using a focused ion beam has been used. In this method, a peripheral portion of a sample is subjected to etching processing so that a portion including a desired observation region inside the sample is left. Then, the remaining portion is subjected to etching processing until the remaining portion has such a thickness that allows an electron beam to transmit therethrough, to thereby form a thin film portion. In this manner, a TEM sample can be prepared with pinpoint accuracy with regard to the portion including the desired observation region.
However, there has been a problem in that, in preparing the TEM sample, if the thickness of the thin film portion becomes smaller, the thin film portion is curved due to internal stress. As a method for solving the problem, there has been proposed a technology of forming a slit in a curved portion so as to release the thin film portion from stress (see JP-A-2000-35391).
In recent years, sizes of the device structure and defects as observation targets have become smaller. Accordingly, in TEM observation, in order to observe only the observation target accurately, it is necessary to prepare a TEM sample having an extremely small thin film portion having a thickness of, for example, 50 nm or less. In this case, there has been a problem in that the thin film portion is bent and curved even when a slit is formed therein as in the above-mentioned related-art technology.